I don't do VEX, but I do have experience with light sensors, which are used to follow lines. FIRST off, do you have it in an area where it has enough light? Not just light from the outside, I also mean light coming to the sensor area. If the sense itself is not receiving enough light, it may not work, and/or it may take longer to find the line, since there isn't enough light to distinguish line from no line.

As for the slowness, do you mean that the line tracking itself is slow, or that it just takes a while to recognize the line, but after it does the robot goes through it quickly.